When I say I don't know...

When I say I don't know / I'm not sure...

It means:
1) I respect you as one of my peers in terms of knowledge and would like to engage in an intellectual conversation with you, willingly and humbly accept any mistakes or misinformed judgement I might have.
2) This might be despite the fact that I know that I'm more well-read and informed than you are.

Don't mistake my respect for your opinion as a sign of weakness.